This invention relates to an improved method for the recovery of oil from subterranean hydrocarbon-bearing formations containing low API gravity, viscous oils or bitumen. More particularly, the invention relates to the production of low-mobility hydrocarbons or bitumen from tar sands utilizing a low-temperature oxidation technique.
The recovery of viscous oils from formations and bitumen from tar sands has generally been difficult. Although some improvement has been realized in stimulating recovery of heavy oils, i.e., oils having an API gravity in the range of 10.degree. to 25.degree. API, little, if any, success has been realized in recovering bitumen from tar sands. Bitumen can be regarded as highly viscous oils having a gravity in the range of about 5.degree. to 10.degree. API and contained in an essentially unconsolidated sand referred to as tar sands.
Vast quantities of tar sands exist in the Athabasca region of Alberta, Canada. While these deposits are estimated to contain over seven hundred billion barrels of oil or bitumen, in-situ recovery therefrom using conventional techniques has not been too successful. The reasons for the lack of success relate principally to the fact that bitumen is extremely viscous at the temperature of the formation, with consequent low mobility. In addition, these tar sand formations have very low permeability, despite the fact they are unconsolidated.
Since it is known that the viscosity of oil decreases markedly with an increase in temperature, thereby improving the mobility of the oil, thermal recovery techniques have been investigated for recovery of bitumen from tar sands. These thermal recovery methods generally include steam injection, hot water injection and in-situ combustion, and the general techniques in employing these methods are well-known in the prior art.
In addition, variations and improvements in the basic techniques are described in the prior art, such as the "huff and puff" method utilizing steam, and the reverse in-situ combustion technique. Improvements have also been set forth in the in-situ combustion method that employ the use of water injection, either simultaneously or intermittently with the air or oxygen-containing gas, to scavenge the residual heat in the formation and also to improve the conformance and sweep efficiency.
Experience has generally shown that these conventional thermal techniques have not been altogether successful when applied to the in-situ recovery of heavy oils. Difficulties have included the build-up of an excessive oil bank ahead of the thermal front that results in plugging of the formation ahead of the front and hence loss of injectivity into the formation. Furthermore, in the case of in-situ combustion as applied to the recovery of heavy oils which contain high percentages of heavy ends, excessive carbonization occurs with a consequent progressive decrease in the rate of movement of the combustion front and the eventual extinguishment of the combustion.
The difficulties become compounded when these techniques are applied to the recovery of bitumen from tar sands since bitumen has a lower API gravity, i.e., 5.degree. to 10.degree. API, and a higher viscosity, i.e., in the millions of centipoises, as compared with heavy oils. In addition, the permeability of the tar sands is so low that difficulty has been experienced in establishing fluid communication within the formation.
Still later developments utilizing thermal methods for the recovery of viscous oils and in particular bitumen from tar sands have sought to overcome the above-recited difficulties. Recent developments relate to the low-temperature oxidation process whreby the temperatures attained in the formation are controlled and maintained well-below those temperatures reached in the conventional in-situ combustion process. In the low-temperature oxidation process, such as that taught in U.S. Pat. No. 4,006,778, issued Feb. 8, 1977, a mixture of an oxygen-containing gas and steam is injected wherein the temperature of the injected mixture corresponds to the temperature of saturated steam at the pressure of the formation. A low-temperature oxidation is caused to occur in the formation whereby the temperature in the formation is raised to the temperature of the injected saturated steam. Preferably this temperature is in the range of 250.degree. to 500.degree. F. Control of the temperature is accomplished by the presence of the saturated steam, that is, by the presence of a liquid water phase. With the rise in temperature, improved mobility of the bitumen, with minimum carbonization, is effected, so that the bitumen can be displaced through th formation toward a production well from which it is produced. The continued injection of the mixture of the oxygen-containing gas and steam provides the displacement means.
Prior art also teaches modifications in the low-temperature oxidation process such as set forth in U.S. Pat. No. 3,978,925 which issued Sept. 7, 1976, wherein a soak period is provided by shutting in the injection and production wells for a period of time to permit the injected oxygen to be consumed in the low-temperature oxidation reaction with hydrocarbons within the formation. Further, U.S. Pat. No. 3,993,132 which issued Nov. 23, 1976, teaches using an optimum gas to steam ratio in the low-temperature oxidation process, which ratio is decreased as the cumulative amount of steam is increased. In yet another development as set forth in U.S. Pat. No. 3,976,137 which issued Aug. 24, 1976, employing the low-temperature oxidation process, the use of a ratio of oxygen to steam in the range of 200 to 800 standard cubic feet of oxygen per barrel of steam (SCF/bbl) or 0.20 to 0.80 MSCF/bbl is taught.
The instant invention discloses an improvement in the low-temperature oxidation process whereby the recovery efficiency is improved by utilizing and maintaining the ratio of the free oxygen in the oxygen-containing gas to the steam at very low ratios.